This invention relates to a microelectromechanical system, and more particularly, to microelectromechanical system having a thermolysis reaction actuating pump.
Microelectromechanical systems, often referred to as MEMs, typically are miniature embedded systems including one or many micro-machined components or structures. They often enable higher level functions, although in and of themselves their utility may be limited. For example, a micro-machine pressure sensor is typically useless by itself, but under the hood of a vehicle, such a pressure sensor can be used to control the fuel air mixture of the engine. MEMs often integrate smaller functions into one package for greater utility. For example, MEMs may be used in an acceleration sensor with an electronic circuit for self diagnostics. MEMs can also provide a cost benefit, directly through lower unit pricing, or indirectly by cutting service and maintenance costs.
Micro-mechanical structures, components and systems are miniature devices that enabled operation of complex systems. MEMs currently have a variety of applications including automotive, medical, consumer, industrial and aerospace. MEMs technology includes a variety of design and fabrication processes, many having their foundation in the semiconductor or integrated circuit industry.
Many microelectromechanical systems utilize sensing and actuation techniques. One of the objectives of sensing devices is to transduce a specific physical parameter into electrical energy. An intermediate conversion step may be required. For example, a pressure or acceleration is converted into mechanical stress, which is then converted into electricity. Temperature measurements use the dependency of various material properties on temperature is a common sensing technique. The characteristic is pronounced in the electrical resistance of metals wherein the temperature coefficient of resistance of most metals ranges between 10 and 100 ppm per xc2x0 C.
Two other common sensing techniques utilize piezoresistivity and piezoelectricity. Doped silicon exhibits piezoresistive behavior and can be utilized for many pressure and acceleration sensor designs. Measuring the change in resistance and amplifying the corresponding output signal is a technique well known to those skilled in the art. However, silicon piezoresistivity is dependent on temperature that must be compensated for with external electronics.
Capacitive sensing utilizes an external physical parameter that changes either the spacing or the relative dielectric constant between two plates of a capacitor. For example, an applied acceleration moves one of the plates closer to the other. For relative humidity sensors, the dielectric is an organic material whose permittivity is a function of the moisture content. Capacitive sensors provide the advantage of requiring very low-power and are relatively stable with temperature. Further, capacitive sensors can be utilized with electrostatic actuation to provide closed loop feedback.
Another sensing techniques utilizes electromagnetic signals to measure a physical parameter. Magnetoresistive sensors are utilized on the read heads of high-density computer disk drives to measure the change in conductivity of the material slab in response to the magnetic field of the storage bit. Another form of electromagnetic transducing uses Faraday""s law to detect motion of a current carrying conductor through a magnetic field.
Once the particular parameter has been sensed and determined to be above a predetermined threshold, it is common for microelectromechanical systems to perform a specific task in response to the sensed parameter. Such a task is typically accomplished by any of a variety of common actuation methods. Typical actuation microelectromechanical systems include electrostatic, piezoelectric, thermal, magnetic, and phase recovery using shaped-memory alloys.
Electrostatic actuation relies on the attractive forces between two plates or elements carrying opposite charges. Two objects with an externally applied potential between them have opposite polarities. Therefore an applied voltage always results in an attractive electrostatic force. Electrostatic actuation can be naturally extended to closed-loop feedback. When sensing circuits detect that two plates of a capacitor are being separated under the effect of an external force (such as acceleration), an electrostatic feedback voltage is immediately applied by the controlled electronics to counteract the disturbance and maintain a fixed capacitance. The magnitude of the feedback voltage then becomes a measure of the disturbing force. This closed loop operation is used in many accelerometers and yaw rate sensors.
Piezoelectric actuation can provide significantly large forces particularly when thick piezoelectric films are utilized. Commercially available piezoceramic cylinders can provide up to a few newtons of force with applied potentials of a few hundred volts. However, thin film piezoelectric actuators can only provide a few millinewtons of force. Both piezoelectric and electrostatic methods offer the advantage of low power consumption because the electric current is very small. Thermal actuation typically requires more power than electrostatic or piezoelectric actuation but can provide actuation forces on the order of hundreds of millinewtons or higher.
There are at least three known approaches to utilizing thermal actuation. The first approach takes advantage of the difference in the coefficient of thermal expansion between two joined layers of the similar materials to cause bending with temperature. One layer expands more than the other as the temperature increases. This results in stress at the interface and consequent bending of the stacked layers. The amount of bending depends on the difference in the coefficient of thermal expansion of the layers in the absolute temperature.
A second approach known as thermopneumatic actuation heats a liquid inside of a sealed cavity. Pressure from expansion or evaporation exerts a force on the cavity walls. The method also depends on the absolute temperature of the actuator.
A third method utilizes the suspension of the beam of the same homogeneous material with one end anchored to a supporting frame of the same material. Heating the beam to the temperature above that of the frame causes a differential expansion of the beam""s free end with respect to the frame. Holding the free end stationery gives rise to force proportional to the beam""s length and the temperature differential. Such an actuator delivers a maximum force with zero displacement and no force when the displacement is maximal. Designs between these two extremes can provide both force and displacement. Typically a system of mechanical linkages can optimize the output of the actuator by trading off forces for displacement or vice versa. In this system the actuation is independent of fluctuations in ambient temperatures because it relies on the differences in the temperature between the beam and the supporting frame.
Electrical current in a conductive element that is located within a magnetic field produces an electromagnetic force in a direction perpendicular to the current and magnetic field. This force is proportional to the current, magnetic field, and the length of the element. These characteristics are utilized in magnetic actuation devices.
Shape-memory alloys provide the highest energy density available for actuation. The shape-memory alloys are a special class of alloys that return to a predetermined shape when heated above a critical transition temperature. The material remembers its original shape after being strained and deformed. When the alloy is below its transition temperature it has a low yield strength and is readily deformed into new permanent shapes. The deformation can be 20 times larger than the elastic deformation with no permanent strain. When heated above its transition temperature, the alloy completely recovers to its original shape through complex changes in its crystal structure. The process generates large forces making shape memory alloys ideal for actuation purposes. In contrast, piezoelectric and electrostatic actuators produce only a fraction of the force available from a shape-memory alloy, but can act much more quickly.
The above actuation techniques and others can be utilized in a variety of devices for microelectromechanical systems. Such devices may include the micro-mechanical resonators, high frequency filters, grating light valve displays, optical switches, thermo-mechanical data storage devices, RE switches and micropumps. Researchers in the microelectromechanical system fields have recently shown a strong interest in micropumps.
There are variety of micropumps known to those skilled in the art. A variety of such pumps are described in Kovacs, Micromachined Transducers Sourcebook, 1998, in chapter 9 which is briefly summarized hereafter. The micropumps have been used for many applications, including transporting reagents, delivering pulsatile flows, generating pressure differences, moving cooling fluids, transporting suspended particles or cells, and a variety of other applications.
The bubble pump is one of the simplest micropumps designs. The bubble pump uses three repetitive formation and collapse of vapor bubbles that are typically formed by local heating of the fluid to be pumped. The fluid may become quite hot locally, however there are several applications in which this approach is very useful. Bubble pumps have been reported to provide 3.1 nl displacement operating at 0.5 Hz.
Membrane pumps are reciprocating devices. There are variety of different driving forces utilized in membrane pumps. It is known to provide electrostatically driven membrane to generate pressure pulses. A variety of valves are often associative at the operation of such pumps.
Thermally driven membrane pumps are also under investigation by those skilled in the art. Typically thermally driven membrane pumps have a membrane that fluctuates with the application of heat thereby changing the volume of a pump cavity causing fluid to move in and out of the cavity through inlet and outlet ports.
Diffuser pumps have inlet and outlet ports with increasing and decreasing cross-sectional area in the direction of the flow and are coupled to a chamber with an oscillating pressure. The kinetic energy or flow velocity of the fluid is transformed into potential energy or pressure in the pump. The efficiency of the process is greater in the diffuser direction than in the nozzle direction. The ports conduct more fluid in the diffuser direction than in the nozzle direction resulting a net pumping action.
A variety of rotary micropumps are known to those skilled in the art. One type such pumps is a jet or impeller type micropump. This micropump includes a thin rotor that pivots about a central pin often magnetically driven. The pump includes a central chamber with inlet and outlet ports. As the rotor rotates under the magnetic force, liquid is moved from the inlet port side to the outlet side of the pump chamber.
Another type of rotary micropump is the magnetically driven gear pump. In this type of pump tight tolerances between the teeth of two gears and the surrounding walls allow for the formation of low-leakage sliding seals. Fluid is pumped by the turning gears with the fluid being carried along the walls of the cavity enclosing the gears.
Electrohydrodynamic pumps have no moving parts but make use of electric fields for pumping. Electrohydrodynamic pumps use the electrical properties of the fluid being pumped. An applied electric field is used to induce charges in the fluid and also to electrostatically move the charges. These electrohydrodynamic pumps require either free charges in the fluid such as ions injected into the fluid by electrochemical reactions (injection-type electrohydrodynamic pumps) or a gradient/discontinuity in conductivity and/or permittivity in the fluids to be pumped. A gradient/discontinuity may be achieved through layered fluids, suspended particles or induced aniostropy and are often referred to as non-injection electrohydrodynamic pumps.
Electroosmotic and electrophoretic also have no moving parts. These types of pumps rely an injection of ions or gradients in conductivity/permittivity with the presence of ions and a suitable solvent. Electroosmotic and electrophoretic pumps can be used to move fluids or ions at flow rates in the range of microns up to millimeters per second for wide range of flow channel cross-sectional areas. Electrophoresis involves the movement of ions relative to solvent molecules under an externally generated electric field.
Electroosmotic pumping is more of a bulk phenomenon than electrophoresis and results in the pumping of an electrically natural fluid. Typically a double layer of opposite charges is formed in the solution near the channel walls and the charges in a double layer can then be moved under the influence of an externally applied electric field. The only region where there is significant concentrations of charges is in a double layer. The thin layer near the channel walls will move and doing so will osmotically draw fluid along with it.
Miniature. ultrasonic and vacuum pumps of also been developed and applied in practical applications by those skilled in the art. Although there are a variety of micropumps available, the present invention provides alternatives to and advantages over the prior art.
One embodiment of the invention includes a thermolysis reaction actuating pump system including a first set of walls defining a channel having a cross-sectional area less than one millimeters squared, and wherein a liquid is received in the channel. A second set of walls define a reaction chamber and a thermolytic body is carried in the reaction chamber. The first set of walls and second set of walls are constructed and arranged to all the flow of gas from the reaction chamber into the channel. A heater is positioned to provide heat to the thermolytic body and to disassociate the thermolytic body to produce gas to pump the liquid through the channel.
In another embodiment of the invention, the thermolytic includes a solid.
In another embodiment of the invention, and the olytic body includes a solid in granular form.
In another embodiment of the invention the thermolytic includes a solid in pellet form.
In another embodiment of the invention the thermolytic includes a solid in powder form.
In another embodiment of the invention the powder includes (NH4) CO3.
In another embodiment of the invention the thermolytic body includes (NH4) CO3 powder.
In another embodiment of the invention the liquid includes water.
In another embodiment of the invention the liquid includes deionized water.
Another embodiment of the invention further includes a silicon substrate and wherein the heater includes a diffused resistor formed in the silicon substrate.
In another embodiment of the invention heater includes a printed resistor.
In another embodiment that heater underlies the reaction chamber.
Another embodiment of the invention further includes a heater block interposed between the heater in the reaction chamber.
Another embodiment of the invention further includes an adhesive layer interposed between the heater block and the second set of walls defining reaction chamber.
In another embodiment of the invention the first set of wall includes a photoresist layer.
In another embodiment of the invention the first set of walls includes polymethyl methylacrylate.
In another embodiment of the invention at least a portion of the first set of walls includes a silicon substrate.
In another embodiment of the invention the channel has a width and a height each less than 500 microns.
In another embodiment of the invention the channel has a width and a height each equal to or less than 300 microns.
Another embodiment of the invention further includes a first substrate and wherein the heater includes a printed resistor formed on the first substrate.
In another embodiment of the invention the thermolytic body includes a substance capable of disassociating upon the application of heat to produce gas.
Another embodiment of the invention includes a method of pumping fluid including providing a pump system having a first set of walls defining a channel and a liquid carried in the channel at a first location, a second set of walls defining a reaction chamber and a thermolytic body carried in the reaction chamber, the first set of walls and second set of walls being constructed and arranged to allow gas to travel from the reaction chamber through the channel, and heating the thermolytic body thereby disassociating the thermolytic body to produce gas in an amount to displace the liquid in the channel to a second location.
In another embodiment of the invention the thermolytic includes a solid.
In another embodiment of the invention the thermolytic includes a powder.
In another embodiment of the invention the thermolytic includes (NH4) CO3 powder.
Another embodiment of the invention further includes cooling the system so that the fluid returns to the first location.
In another embodiment of the invention the thermolytic body is heated to a temperature at least as great as 50xc2x0 C.
Another embodiment of the invention the thermolytic body is heated to a temperature at least as great as 56xc2x0 C.
In another embodiment of the invention the thermolytic body is heated to a temperature at least as great as 64xc2x0 C.
In another embodiment of the invention the liquid includes deionized water.